Usually, an ophthalmic lens comprises a visual correction which is determined by a prescription established for the wearer of the lens.
Such a prescription notably indicates an optical power value and an astigmatism value suitable for correcting a wearer for far vision. These values are usually obtained by combining the anterior face of the lens with a posterior face which is generally spherical or sphero-toroidal. For a progressive lens, at least one of the two faces of the lens exhibits variations of sphere and of cylinder, from which result variations of optical power and of astigmatism between different directions of observation through the lens. In particular, the optical power difference between the two points dedicated to the far vision and to the near vision is called the optical addition, and its value must also correspond to the value prescribed for a presbyopic wearer.
Commonly, a progressive lens is manufactured in two successive steps. The first step consists in manufacturing a semi-finished lens, the anterior face of which may exhibit variations of sphere and of cylinder defined in relation to the optical performance levels desired for the finished lens. It is executed in a factory, for example by molding or injection. The semi-finished lenses are divided up into a plurality of articles, which may differ, notably, by the base, by the distribution of the spheres and cylinders of the anterior face, or by the addition. The base is the mean sphere at the point of the lens corresponding to the far vision. The vertical and horizontal distances between the near and far vision points, the respective widths of the areas of the lens corresponding to the near vision and to the far vision, the refractive index of the transparent material which constitutes the semi-finished lens, etc., may also differ from one article to another. Each combination of these characteristics corresponds to a different semi-finished lens article.
The second step consists in manufacturing the posterior face of the lens.
The posterior face of the lens may comprise variations of sphere and of cylinder such that the association of the two faces produces the desired optical performance levels.
The optical surfaces may, after manufacture, exhibit defects of form, in particular the optical surfaces exhibiting addition between two points of this surface may exhibit surface addition defects. Such a defect is generally due to a step of polishing the surface of the lens or of the mold, the polishing not uniformly removing the material from the surface to be polished thus deforming said surface.
The optical function of the final lens can be very sensitive to these surface defects. There is therefore a need for a method that makes it possible to reduce these surface defects.
A known method consists in producing the optical surface first, by measuring for example the surface addition defect, and remanufacturing the same optical surface to which the negative of the defect measured on the first manufactured surface will previously have been added. Thus, the second surface produced will be closer to the theoretical nominal surface. The drawback of such a method is that it entails manufacturing two surfaces to arrive at a satisfactory surface.
Another known method consists in producing a power adjustment at the far vision point. This adjustment consists of producing a certain number of lenses, by measuring the power defect at the far vision point, in other words the deviation between the power at the far vision point obtained relative to the far vision point of the nominal and providing a correction of this power by adding a spherical surface to the entire nominal surface before producing it. This method makes it possible to adjust a local value on the basis of a defect observed on a certain number of lenses or surfaces for which the semi-finished version used is common. However, this criterion is not entirely satisfactory; in practice, the polishing step does not uniformly impact on the entirety of the surface.